Novel algicidal evidence of a bacterium Bacillus sp. LP-10 killing Phaeocystis globosa,a harmful algal bloom causing species

While searching for effective bio-agents to control harmful algal blooms (HABs), the bacterial strain LP-10, which has strong algicidal activity against Phaeocystis globosa (Prymnesiophyceae), was isolated from surface seawater samples taken from the East China Sea. 16S rDNA sequence analysis and morphological characteristics revealed the strain LP-10 belonged to the genus Bacillus. The lytic effect of Bacillus sp. LP-10 against P. globosa was both concentration- and time-dependent. Algicidal activities of different growth stages of the bacterial culture varied significantly. The lytic effect of different parts of the bacterial cultures indicated that the algal cells were lysed by algicidal active compounds in the cell-free filtrate. Analysis of the properties of the active compounds showed that they had a molecular weight of less than 1000 Da and that the active compounds were stable between −80 and 121 °C. The algicidal range assay indicated that five other algal species were also suppressed by strain LP-10, including: Alexandrium catenella, A. tamarense, A. minutum, Prorocentrum micans and Asterionella japonica. Our results suggested that the algicidal bacterium Bacillus sp. LP-10 could be a potential bio-agent to control the blooms of harmful algal species.     

Toxic Effect of a Marine Bacterium on Aquatic Organisms and Its Algicidal Substances against Phaeocystis globosa

Harmful algal blooms have caused enormous damage to the marine ecosystem and the coastal economy in China. In this paper, a bacterial strain B1, which had strong algicidal activity against Phaeocystis globosa, was isolated from the coastal waters of Zhuhai in China. The strain B1 was identified as Bacillus sp. on the basis of 16S rDNA gene sequence and morphological characteristics. To evaluate the ecological safety of the algicidal substances produced by strain B1, their toxic effects on marine organisms were tested. Results showed that there were no adverse effects observed in the growth of Chlorella vulgaris, Chaetoceros muelleri, and Isochrystis galbana after exposure to the algicidal substances at a concentration of 1.0% (v/v) for 96 h. The 48h LC₅₀ values for Brachionus plicatilis, Moina mongolica Daday and Paralichthys olivaceus were 5.7, 9.0 and 12.1% (v/v), respectively. Subsequently, the algicidal substances from strain B1 culture were isolated and purified by silica gel column, Sephadex G-15 column and high-performance liquid chromatography. Based on quadrupole time-of-flight mass spectrometry and PeakView Software, the purified substances were identified as prolyl-methionine and hypoxanthine. Algicidal mechanism indicated that prolyl-methionine and hypoxanthine inhibited the growth of P. globosa by disrupting the antioxidant systems. In the acute toxicity assessment using M. mongolica, 24h LC₅₀ values of prolyl-methionine and hypoxanthine were 7.0 and 13.8 g/L, respectively. The active substances produced by strain B1 can be considered as ecologically and environmentally biological agents for controlling harmful algal blooms.     

Algicidal Activity of Thiazolidinedione Derivatives Against Harmful Algal Blooming Species

Thiazolidinedione (TD) derivatives exhibit algicidal activity against harmful algal blooming species such as Chattonella marina, Heterosigma akashiwo, and Cochlodinium polykrikoides, as reported previously. In this study, the efficacies and selectivities of TD derivatives were tested by analyzing the structure–activity relationships of various TD derivatives. To investigate structure–activity relationships for growth inhibition of harmful algae, we added a methylene group between the cyclohexyl ring and oxygen of 5-(3-chloro-4-hydroxybenzylidene)-TD, which decreased the inhibitory potency of compound 17. Interestingly, another addition of a methylene group significantly increased the inhibitory potency against C. polykrikoides. The addition of 1 μM compound 17 resulted in the cell rupture of harmful algae after less than 10 h incubation at 20 °C. Compound 17 was applied to both harmful and non-harmful algae and showed a drastic reduction in the efficiency of photosystem II, resulting in reduced photosynthetic oxygen evolution. Compound 17 at a 5 μM concentration destroyed all of the harmful algae, while algicidal activity against non-harmful algae did not exceed 30% of the control within the concentration range tested. In contrast, a herbicide, 3-(3,4-dichlorophenyl)-1,1-dimethylurea, tested at a 5 μM concentration, exhibited 40–70% algicidal activity relative to that of the control against both harmful and non-harmful algae. Compound 17 is a promising lead compound for the development of algicides to control harmful algal blooming species.     

Isolation and characterization of a marine algicidal bacterium against the harmful raphidophyceae <Emphasis Type="Italic">Chattonella marina</Emphasis>

A bacterial strain named AB-4 showing algicidal activity against Chattonella marina was isolated from coastal water of ULjin, Republic of Korea. The isolated strain was identified as Bacillus sp. by culture morphology, biochemical reactions, and homology research based on 16S rDNA. The bacterial culture led to the lysis of algal cells, suggesting that the isolated strain produced a latent algal-lytic compound. Amongst changes in algicidal activity by different culture filtrate volumes, the 10% (100 μl/ml) concentration showed the biggest change in algicidal activity; there, estimated algicidal activity was 95%. The swimming movements of Chattonella marina cells were inhibited because of treatment of the bacterial culture; subsequently, Chattonella marina cells became swollen and rounded. With longer exposure time, algal cells were disrupted and cellular components lost their integrity and decomposed. The released algicide(s) were heat-tolerant and stable in pH variations, except pH 3, 4, and 5. Culture filtrate of Bacillus sp. AB-4 was toxic against harmful algae bloom (HAB) species and nontoxic against livefood organisms. Bacillus sp. AB-4 showed comparatively strong activity against Akashiwo sanguinea, Fibriocapsa japonica, Heterosigma akashiwo, and Scrippsiella trochoidea. These results suggest that the algicidal activity of Bacillus sp. AB-4 is potentially useful for controlling outbreaks of Chattonella marina.     

Algicidal metabolites produced by Bacillus sp. strain B1 against Phaeocystis globosa

The bloom of Phaeocystis globosa has broken out frequently in the coastal areas of China in recent years, which has led to substantial economic losses. This study shows that Bacillus sp. strain B1, which was previously identified by our group, is effective in regulating P. globosa by excreting active metabolites. Heat stability, pH stability and molecular weight range of the algicidal compounds from strain B1 were measured and the results demonstrated that the algicidal activities of these compounds were not affected by pH or temperature variation. The algicidal compounds extracted with methanol were isolated and purified by ODS-A column chromatography and HPLC. The algicidal compounds corresponding to peaks 2–5 eluted from HPLC were further analysed by quadrupole time-of-flight mass spectrometry (Q-TOF–MS). PeakView? Software determined the compounds corresponding to peaks 2–5 to be l-histidine, o-tyrosine, N-acetylhistamine and urocanic acid on the basis of the accurate mass information, the isotopic pattern and MS–MS spectra. Furthermore, these compounds were also able to eliminate Skeletonema costatum, Prorocentrum donghaiense and Heterosigma akashiwo. This is the first report of bacteria-derived algicidal compounds being identified only by Q-TOF–MS and PeakView? Software, and these compounds may be used as the constituents of algicides in the future.     

Isolation and Identification of Algicidal Compound from Streptomyces and Algicidal Mechanism to Microcystis aeruginosa

The biological control of cyanobacterial harmful algal blooms (cyanoHABs) is important to promote human health, environmental protection, and economic growth. Active algicidal compounds and algicidal mechanisms should be identified and investigated to control cyanoHABs. In this study, the algicidal actinobacterium Streptomyces sp. L74 was isolated from the soil of a nearby pond which located in the center lake of Guanghzou Higher Education Mega Center. Results showed that the algicidal activities of cyanoHABs are mainly achieved via an indirect attack by producing algicidal compounds. All active algicidal compounds are hydrophilic substances that are heat and pH stable. In the present study, an active compound (B3) was isolated and purified by high-performance liquid chromatography and identified as a type of triterpenoid saponin (2-hydroxy-12-oleanene-3, 28-O-D-glucopyranosyl) with a molecular formula of C₄₂H₇₀O₁₃ as determined by infrared spectrometry, electrospray ionization mass spectrometry, and nuclear magnetic resonance. Active algicidal compounds from Streptomyces sp. L74 were shown to disrupt the antioxidant systems of Microcystis aeruginosa cells.     

A marine bacterium producing protein with algicidal activity against Alexandrium tamarense

Interactions between bacteria and harmful algal bloom (HAB) species have been acknowledged as an important factor of regulating the population of these algae. In the study, two strains of algicidal bacteria, DHQ25 and DHY3, were screened out because of their probably secreting algicidal proteins against axenic Alexandrium tamarense. Molecular characterization classified them to the γ-proteobacteria subclass and to the genus Vibrio and Pseudoalteromonas, respectively. After centrifugation and ultrafiltration, chromatography of the cultural supernatants of DHQ25 revealed 8 peaks by HPLC. SDS-PAGE and Native PAGE determination showed that peak 7 to be a monoband peak. Both xenic and axenic culture of A. tamarense were susceptible to the purified protein (short for P7 below) indicated by algicidal activity assay. Observation of algicidal process demonstrated that algal cells were lysed and cellular substances were released under visual fields of microscope. P7 proved to be a challenge controller of A. tamarense by the above characterizations of algicidal activity assaying and algicidal process. This is the first report of a protein algicidal to the toxic dinoflagellate A. tamarense. The findings increase our knowledge of bacterial–algal interactions and the role of bacteria during controlling HABs.     

Novel rhodanine derivatives are selective algicides against <Emphasis Type="Italic">Microcystis aeruginosa</Emphasis>

In this study, a series of rhodanine derivatives containing various substituents was synthesized and tested for in vitro algicidal activity. Among the tested substituent groups, phenyl substituents with halogen groups showed good inhibitory potency. Furthermore, the compound with chlorine at the C2 position of the phenyl ring exhibited a higher algicidal effect than the compound with chlorine at the C3 position of the phenyl ring. Among the various rhodanine derivatives tested, 5-(2,4-dichlorobenzylidene)- rhodanine (compound 20) was the most potent inhibitor against M. aeruginosa with a lethal concentration 50 (LC₅₀) value of 0.65 μM and Selenastrum capricornutum with an LC₅₀ value of 0.82 μM. To verify the feasibility of their use in ecosystems, 25 h of acute ecotoxicity tests were carried out for three derivatives against Danio rerio and Daphnia magna. No mortality was observed in groups exposed to 2.0 μM of compound 20 after 100 h of exposure. Moreover, a survival rate of 100% was observed in D. magna exposed to 2 μM of compound 20 for 100 h. Overall, the results show that rhodanine derivatives are a possible method for controlling and inhibiting harmful algal blooms.     

Marine bacteria antagonistic to the harmful algal bloom species Alexandrium tamarense (Dinophyceae)

As part of efforts to enhance the strategies employed to manage and mitigate algal blooms and their adverse effects, algicidal bacteria have shown promise as potential suppressors of these events. Nine strains of bacteria algicidal against the toxic dinoflagellate, Alexandrium tamarense, were isolated from the East Sea area, China. Sequence analysis of 16S rDNA showed that all the algicidal bacteria belonged to the γ-proteobacteria subclass and the genera Pseudoalteromonas (strain SP31 and SP44), Alteromonas (strain DH12 and DH46), Idiomarina (strain SP96), Vibrio (strain DH47 and DH51) and Halomonas (strain DH74 and DH77). To assess the algicidal mode of these algicidal bacteria, bacterial cells and the filtrate from bacterial cultures were inoculated into A. tamarense cultures, and fluorescein diacetate vital stain was applied to monitor the growth of the algal cells. The results showed that all the algicidal bacteria exhibited algicidal activity through an indirect attack since algicidal activity was only detected in cell free supernatants but not the bacterial cells. This is the first report of bacteria from the genus Idiomarina showing algicidal activity to the toxic dinoflagellate A. tamarense and these findings would increase our knowledge of bacterial–algal interactions and the role of bacteria during the population dynamics of HABs.     

Algicidal Effects of a Novel Marine Pseudoalteromonas Isolate (Class Proteobacteria,Gamma Subdivision) on Harmful Algal Bloom Species of the Genera Chattonella,Gymnodinium, and Heterosigma

During a bacterial survey of the Huon Estuary in southern Tasmania, Australia, we isolated a yellow-pigmented Pseudoalteromonas strain (class Proteobacteria, gamma subdivision), designated strain Y, that had potent algicidal effects on harmful algal bloom species. This organism was identified by 16S rRNA sequencing as a strain with close affinities to Pseudoalteromonas peptidysin. This bacterium caused rapid cell lysis and death (within 3 h) of gymnodinoids (including Gymnodinium catenatum) and raphidophytes (Chattonella marina and Heterosigma akashiwo). It caused ecdysis of armored dinoflagellates (e.g., Alexandrium catenella, Alexandrium minutum, and Prorocentrum mexicanum), but the algal cultures then recovered over the subsequent 24 h. Strain Y had no effect on a cryptomonad (Chroomonas sp.), a diatom (Skeletonema sp.), a cyanobacterium (Oscillatoria sp.), and two aplastidic protozoans. The algicidal principle of strain Y was excreted into the seawater medium and lost its efficacy after heating. Another common bacterial species, Pseudoalteromonas carrageenovora, was isolated at the same time and did not have these algicidal effects. The minimum concentrations of strain Y required to kill G. catenatum were higher than the mean concentrations found in nature under nonbloom conditions. However, the new bacterium showed a chemotactic, swarming behavior that resulted in localized high concentrations around target organisms. These observations imply that certain bacteria could play an important role in regulating the onset and development of harmful algal blooms.Historically, the dynamics of marine bacterial and algal populations have been studied largely in isolation. Increasing evidence is now pointing toward a close spatial and temporal association between the two and recently attention has been focused on phagocytosis of bacteria by photosynthetic flagellates (21, 28, 30). In contrast, the importance of inhibitory or predatory bacteria in regulating populations of different algal species has received relatively little attention (9, 11). Some bacteria may selectively promote bloom formation by algal species (13), while other bacteria have algicidal effects and are involved in the termination and decomposition of algal blooms (12). The latter finding has raised the possibility of bacterial control of harmful algal blooms (19). There is little data on the occurrence of marine algicidal bacteria outside Japan, where toxic blooms are frequent events (20), and algicidal bacteria have been isolated during toxic blooms of naked dinoflagellates and raphidophytes (9).Gymnodinium catenatum (a causative organism of paralytic shellfish poisoning) is thought to have been introduced into southern Tasmania via ballast water after 1973, and in some years it has a severe negative impact on the shellfish industry (16). Previous efforts to understand and predict the seasonal and interannual variability of harmful algal blooms have largely focused on the environmental factors that affect dinoflagellate growth in the water column, notably water temperature, rainfall, and water column stability (16). Rainfall and estuarine flow patterns also largely determine the allochthonous input of dissolved organic matter (DOM), which is a source of organic carbon for bacteria (27) and is possibly involved in micronutrient dynamics that promote G. catenatum growth (3, 6). As part of a study investigating DOM, bacteria, and algal interactions in the Huon Estuary (24), we isolated two bacterial strains that we tested for possible alga-bacterium interactions by using cultures of G. catenatum. Both bacteria appeared to be Pseudoalteromonas species, which are extremely common, slightly halophilic, gram-negative bacteria found in many marine ecosystems. Preliminary observations indicated that one of the strains was extremely toxic towards G. catenatum, while the other was more benign. The aims of this study were (i) to determine the taxonomic identity of the bacteria, (ii) to document by light microscopy the sequence of algal cell lysis after exposure to an algicidal Pseudoalteromonas strain and compare this lysis to the effect of the more benign Pseudoalteromonas species, (iii) to define the minimum bacterial concentrations required for algicidal effects and compare these concentrations to concentrations in natural water samples, and (iv) to investigate the range of potential target organisms for the bacterium.     

A novel marine bacterium algicidal to the toxic dinoflagellate Alexandrium tamarense

Aims: This work is aiming at investigating algicidal characterization of a bacterium isolate DHQ25 against harmful alga Alexandrium tamarense. Methods and Results: 16S rDNA sequence analysis showed that the most probable affiliation of DHQ25 belongs to the γ‐proteobacteria subclass and the genus Vibrio. Bacterial isolate DHQ25 showed algicidal activity through an indirect attack. Xenic culture of A. tamarense was susceptible to the culture filtrate of DHQ25 by algicidal activity assay. Algicidal process demonstrated that the alga cell lysed and cellular substances released under the visual field of microscope. DHQ25 was a challenge controller of A. tamarense by the above characterizations of algicidal activity assay and algicidal process. Conclusion: Interactions between bacteria and harmful algal bloom (HAB) species proved to be an important factor regulating the population of these algae. Significance and Impact of Study: This is the first report of a Vibrio sp. bacterium algicidal to the toxic dinoflagellate A. tamarense. The findings increase our knowledge of the role of bacteria in algal–bacterial interaction.     

ALGICIDAL BACTERIA ACTIVE AGAINST GYMNODINIUM BREVE (DINOPHYCEAE). I. BACTERIAL ISOLATION AND CHARACTERIZATION OF KILLING ACTIVITY1,3

Interactions between bacteria and species of harmful and/or toxic algae are potentially important factors affecting both the population dynamics and the toxicity of these algae. Recent reports of bacteria lethal to certain harmful algal bloom (HAB) species, coupled with a rapidly evolving interest in attempting to minimize the adverse effects of HABs through various prevention, control, and mitigation strategies, have focused attention on defining the role of algicidal bacteria in bloom termination. The aim of the present study was to determine whether algicidal bacteria active against Gymnodinium breve Davis, a dinoflagellate responsible for frequent and protracted red tides in the Gulf of Mexico, are present in the waters of the west Florida shelf. To date, we have isolated two bacterial strains from this region lethal to G. breve and have begun to characterize the algicidal activity of one of these strains, 41-DBG2. This bacterium, a yellow-pigmented, gram-negative rod, was isolated from waters containing no detectable G. breve cells, suggesting that such bacteria are part of the ambient microbial community and are not restricted to areas of high G. breve abundance. Strain 41-DBG2 produced a dissolved algicidal compound(s) that was released into the growth medium, and the algicide was effective against the four Gulf of Mexico G. breve isolates tested as well as a closely related HAB species that also occurs in this region, Gymnodinium mikimotoi Miyake et Kominami ex Oda. Nonetheless, data showing that a nontoxic isolate of Gymnodinium sanguineum Hirasaka from Florida Bay was not affected indicate that the algicidal activity of this bacterium does exhibit a degree of taxonomic specificity. Our efforts are currently being directed at resolving several critical issues, including the identity of the algicide(s), the mechanisms regulating its production and ability to discriminate between target algal species, and how the growth rate of 41-DBG2 is affected by the presence of G. breve cells. We have also proposed a conceptual model for interactions between algicidal bacteria and their target species to serve as a testable framework for ensuing field studies.     

Enhanced species-specific chemical control of harmful and non-harmful algal bloom species by the thiazolidinedione derivative TD49

Culture experiments involving 23 algae strains were conducted to evaluate the algicidal effects of a newly developed algicidal thiazolidinedione (TD) derivative (TD49) on non-harmful and harmful algal bloom (HAB) species. We also assessed the effect of various concentrations of TD49 on various growth phases (lag, logarithmic, and stationary) of the harmful algae Heterocapsa circularisquama (Dinophyceae) and Heterosigma akashiwo (Raphidophyceae; hereafter, Heterosigma) and the non-harmful diatoms Skeletonema costatum and Chaetoceros didymus. The inhibitory ratios (%) for H. circularisquama and Heterosigma at 2.0 μM TD49 were significantly higher than those at other concentrations, and the inhibitory ratio varied depending on growth phase and species as follows: logarithmic?≥?stationary?>?lag phase for H. circularisquama and logarithmic?≥?lag?>?stationary phase for Heterosigma. Although the inhibitory ratios for C. didymus were similar to those for the two harmful algae (H. circularisquama and Heterosigma), inhibitory effects on S. costatum were not apparent at >2.0 μM in any growth phase. The algicidal activity of TD49 on the harmful and non-harmful algae was as follows: unarmored HAB species?>?armored HAB species?>?diatom species?>?cryptophyte species. TD49 was algicidal to most HABs but had a little inhibitory effect on some non-harmful algae, implying that TD49 has selective algicidal activity. Our results indicate that TD49 is potentially of use in the control of HAB species within semi-enclosed bays.     

Characterization of an algicidal bacterium Brevundimonas J4 and chemical defense of Synechococcus sp. BN60 against bacterium J4

As part of efforts to enhance the strategies explored to eliminate the adverse impacts of cyanobacterial blooms, we isolated an algicidal bacterium, J4, from Lake Taihu. Analysis of 16S rDNA sequence revealed that strain J4 belonged to the genus Brevundimonas. Bacterium J4 exhibited algicidal activity mainly through excretion of extracellular algicidal compounds that were further extracted with methanol and purified by silica gel chromatography and high performance liquid chromatography (HPLC). The compounds showed thermal stability, strong polarity and water solubility in J4 cultures. Study on the algicidal activity of J4 against two dominant cyanobacterial bloom-forming species in Lake Taihu showed that J4 exhibited lower algicidal rate against Synechococcus sp. BN60 (48.6%, t = 6 days) than against Microcystis aeruginosa 9110 (91.8%, t = 6 days). Additionally, rapid reduction in cell density of J4 was observed in co-cultures of Synechococcus sp. BN60 and bacterium J4 but not observed in co-cultures of M. aeruginosa 9110 and bacterium J4 during algicidal process, which was the main reason why the algicidal rate of J4 against BN60 was lower than against 9110. The reduction in cell density of J4 resulted from inducible production of antimicrobial-like compound secreted by Synechococcus sp. BN60 in co-cultures of Synechococcus sp. BN60 and bacterium J4, which reflected a kind of chemical defense from cyanobacteria (BN60) against algicidal bacteria (J4). However, M. aeruginosa 9110 had no chemical defense against J4, suggesting that whether cyanobacterial chemical defense occurs or not between cyanobacteria and algicidal bacteria depends on specific cyanobacteria–algicidal bacteria pairs. These results show that not only one-sided algicidal effect but also two-sided reciprocal inhibition interactions exist between algicidal bacteria and cyanobacteria, indicating the complexity of cyanobacteria–algicidal bacteria interactions in Lake Taihu and the need to take the cyanobacterial defensive responses into consideration when assessing potential use of algicidal bacteria.     

Algicidal Effects of a Novel Marine Actinomycete on the Toxic Dinoflagellate <Emphasis Type="Italic">Alexandrium tamarense</Emphasis>

A marine actinomycete strain BS01 with algicidal activity to the toxic dinoflagellate, Alexandrium tamarense, was isolated from Xiamen Bay, China. Sequence analysis of 16S rDNA demonstrates that BS01 is closely related to the genus Brevibacterium of Actinomycetales. BS01 exhibited algicidal activity in an indirect manner. Additional organic nutrients, but not algal-derived dissolved organic matter, were necessary for the synthesis of yet unidentified algicidal compounds (molecular weight less than 100), which were heat tolerant, a stable in acidic or alkali conditions, and exhibited a wide range of algicidal activity. This is the first report of an actinomycete algicide to the toxic dinoflagellate A. tamarense. Our results indicate that BS01 could be a potential bio-agent for controlling harmful algal blooms.     

Isolation and Phylogenetic Analysis of Novel Viruses Infecting the Phytoplankton Phaeocystis globosa (Prymnesiophyceae)

Viruses infecting the harmful bloom-causing alga Phaeocystis globosa (Prymnesiophyceae) were readily isolated from Dutch coastal waters (southern North Sea) in 2000 and 2001. Our data show a large increase in the abundance of putative P. globosa viruses during blooms of P. globosa, suggesting that viruses are an important source of mortality for this alga. In order to examine genetic relatedness among viruses infecting P. globosa and other phytoplankton, DNA polymerase gene (pol) fragments were amplified and the inferred amino acid sequences were phylogenetically analyzed. The results demonstrated that viruses infecting P. globosa formed a closely related monophyletic group within the family Phycodnaviridae, with at least 96.9% similarity to each other. The sequences grouped most closely with others from viruses that infect the prymnesiophyte algae Chrysochromulina brevifilum and Chrysochromulina strobilus. Whether the P. globosa viruses belong to the genus Prymnesiovirus or form a separate group needs further study. Our data suggest that, like their phytoplankton hosts, the Chrysochromulina and Phaeocystis viruses share a common ancestor and that these prymnesioviruses and their algal host have coevolved.     

Isolation and characterization of a marine algicidal bacterium against the toxic dinoflagellate Alexandrium tamarense

Interactions between bacteria and harmful algal bloom (HAB) species have been acknowledged as an important factor regulating both the population dynamics and toxin production of these algae. A marine bacterium SP48 with algicidal activity to the toxic dinoflagellate, Alexandrium tamarense, was isolated from the Donghai Sea area, China. Genetic identification was achieved by polymerase chain reaction amplification and sequence analysis of 16S rDNA. Sequence analysis showed that the most probable affiliation of SP48 was to the γ-proteobacteria subclass and the genus Pseudoalteromonas. Bacterial isolate SP48 showed algicidal activity through an indirect attack. Additional organic nutrients but not algal-derived DOM was necessary for the synthesis of unidentified algicidal compounds but β-glucosidase was not responsible for the algicidal activity. The algicidal compounds produced by bacterium SP48 were heat tolerant, unstable in acidic condition and could be easily synthesized regardless of variation in temperature, salinity or initial pH for bacterial growth. This is the first report of a bacterium algicidal to the toxic dinoflagellate A. tamarense and the findings increase our knowledge of bacterial–algal interactions and the role of bacteria during the population dynamics of HABs.     

Isolation and algicidal characterization of <Emphasis Type="Italic">Bowmanella denitrificans</Emphasis> S088 against <Emphasis Type="Italic">Chlorella vulgaris</Emphasis>

One strain of algicidal bacterium, named as S088, was isolated from the intestine of healthy sea cucumbers (Stichopus horrens) in the South China Sea. Based on the analysis of its biochemical characteristics and 16S rDNA gene sequence, S088 was identified as Bowmanella denitrificans. Importantly, the algicidal activity of S088 on Chlorella vulgaris was characterized in this study. The initial densities of bacterial and algal cell showed strong influence on the removal rates of chlorophyll a. When the strain S088 was cultured under a complete darkness condition at 30 °C, its algicidal activity reached the highest level. Furthermore, it was found that the filtered supernatant from bacterial cultures had full algicidal activity, suggesting that the secreted compounds from S088 are involved in the observed algicidal action of S088. Moreover, the algicidal compounds were heat tolerant and had no cytotoxicity against fish cells, indicating that S088 would have a promising application as a safe probiotics for S. horrens. Finally, this is the first report about the algicidal activities in B. denitrificans.